1. Field of the Invention
This invention is directed to a method and apparatus for forming thermal barrier coatings having improved performance characteristics for high temperature applications, and specifically to an electron beam physical vapor deposition process for applying thermal barrier coatings having lower thermal conductivity.
2. Discussion of Prior Art
Physical vapor deposition methods are used to apply coatings to various workpieces. These coatings, typically ceramic material, provide the workpiece, such as an airfoil when placed in the hot gas flow path insulation from hot combustion gases such as are found in a turbine engine. It is important for the coating to have low thermal conductivity and spallation resistance in order to provide the desired benefits. The thermal conductivity of the coating critically controls the thickness requirements needed to achieve the required temperature benefit for a particular design. However, the coating on the airfoil adds mass that subsequently affects stresses in the airfoil and the life of the underlying substrate. It is thus desirable to minimize the density and thickness of the coating while maintaining the required thermal characteristics of the coating.
Methods for depositing these ceramic coatings entail placing a source of material in a vacuum chamber. The workpiece is placed in proximity to the material source within the vacuum chamber, typically over the material source. A high energy source, such as an electron gun, emits a high energy beam directed at the material source, which melts and vaporizes material from the source and disperses vapors, some of which are deposited onto the workpiece to form a coating.
One such method is set forth in U.S. Pat. No. 5,418,003 (""003) to Bruce et al. and assigned to the assignee of the present invention. The ""003 patent is directed to a method of improving the coating resulting from an electron beam physical vapor deposition process by eliminating gases present in the ingot, the material source as a preparatory step for the deposition process.
Another method is set forth in U.S. Pat. No. 5,698,273 (""273) to Azad et al. and also assigned to the assignee of the present invention. In the ""273 patent, a plurality of electron beams are directed at the source of the material, the ingot, to improve the vaporization rate of material from the melt pool. This method discloses placing the workpiece in a vacuum chamber above a crucible containing the ingot. A primary electron beam scans the surface of the ingot to develop a melt pool on the surface of the ingot and vaporize ingot material. A secondary electron beam is directed at the melt pool to increase the vaporization rate of material from the melt pool.
The traditional method of applying a coating by this method has been to position a workpiece or a plurality of workpieces directly over the ingot or material source(s), applying energy until the desired coating thickness was achieved. In systems with a plurality of ingot sources, the spacing of the ingots and the height of the specimens above the plane containing the ingots are adjusted to maximize the number of parts that can be coated in one cycle with a uniform layer of coating material. The coatings produced in this manner have had a relatively uniform thickness, uniform thermal conductivity and have performed substantially identically in service. This prior art practice has positioned the articles undergoing a physical vapor deposition coating operation at locations over the ceramic ingot having relatively fixed distances. Coatings applied to articles within this narrow range of locations have been relatively uniform with respect to both thickness and thermal conductivity. This uniformity and reproducibility is a desirable feature for predicting subsequent performance of the coated articles. Typically, these physical vapor deposition methods have been used to apply consistent thermal barrier coatings that have predictable behavior in service, and improvements in the thermal performance of the coatings have generally been accomplished by increasing the thickness of the coating.
An improved method for applying a ceramic material, such as a thermal barrier coating to an article such as a turbine airfoil is provided by the present invention. The effectiveness of a thermal barrier coating can be improved by reducing its thermal conductivity. The present invention provides a method for applying a ceramic material as a coating to a substrate article in which the thermal conductivity of the ceramic material is reduced or lowered. The process utilizes typical physical vapor deposition apparatus that includes a vacuum chamber, a vacuum pump for evacuating the vacuum chamber, two or more sources of ceramic material that is to be applied as a coating on the article and a high energy source to provide sufficient energy to melt and vaporize the material that forms the coating. The coating with the improved or lowered thermal conductivity is formed by placing the article to be coated in a vacuum chamber at a preselected distance away from the source of material that forms the coating. The sources of material are typically two or more ceramic ingots.
It has been discovered that the thermal conductivity of a coating applied by a physical vapor deposition (PVD) method is dependent upon its distance from the source(s) of material used for the coating. The thermal conductivity of the applied coating can be altered by adjusting the position of the article undergoing the PVD process with respect to the ingot(s). Increasing the distances of the article or workpiece from the ingot or source of ceramic provides a coating of lower thermal conductivity. In accordance with the present invention, the article to be coated is positioned at a distance required to achieve at least a 10% reduction in the thermal conductivity of the applied coating.
An advantage of the present invention is that a coated article can be produced having a lower conductivity and, hence capable of withstanding higher temperature gradients with the same applied thickness.
Another advantage of the present invention is that an article having a thinner coating but capable of withstanding the same temperature gradients as articles coated by prior practices can be produced using the teachings of the present invention.
Still another advantage of the present invention is that a series of articles having the same thickness, but with a variety of temperature capabilities can be produced simultaneously by adjusting the positions of the articles with respect to the source.
The manufacturing process of the present invention can be modified by separating the ingots in a system including at least two ingots such that the thickness variation can be tailored to a thermal conductivity variation to obtain the desired temperature gradient. Such a processing change provides the advantage of allowing an increase in the number of parts having a predictable thermal gradient value coated at one time.
An advantage of the present invention is that existing PVD processing equipment can be utilized to perform the processes of the present invention with minor modifications to the equipment in most cases.